Method of correcting vision

ABSTRACT

An ophthalmic laser system includes a laser beam delivery system and an eye tracker responsive to movement of the eye operable with a laser beam delivery system for ablating corneal material of the eye through placement of laser beam shot on a selected area of the cornea of the eye. The shots are fired in a sequence and pattern such that no laser shots are fired at consecutive locations and no consecutive shots overlap. The pattern is moved in response to the movement of the eye.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of and incorporates byreference application Ser. Nos. 09/376,133, filed Aug. 17, 1999, whichis a continuation of application Ser. No. 08/232,615, filed Apr. 25,1994, now issued as U.S. Pat. No. 5,980,513, and further incorporatesU.S. Pat. Nos. 5,849,006 and 5,632,742 by reference, all of which arecommonly owned and have the disclosures incorporated by reference.

FIELD OF THE INVENTION

[0002] The invention relates generally to laser systems, and moreparticularly to a laser system used to erode a moving surface such as aneye's corneal tissue.

BACKGROUND OF THE INVENTION

[0003] Use of lasers to erode all or a portion of a workpiece's surfaceis known in the art. In the field of ophthalmic medicine,photorefractive keratectomy (PRK) is a procedure for laser correction offocusing deficiencies of the eye by modification of corneal curvature.PRK is distinct from the use of laser-based devices for more traditionalophthalmic surgical purposes, such as tissue cutting or thermalcoagulation. PRK is generally accomplished by use of a 193 nanometerwavelength excimer laser beam that ablates away the workpiece, i.e.,corneal tissue, in a photo decomposition process. Most clinical work tothis point has been done with a laser operating at a fluence level of120-195 mJ/cm² and a pulse-repetition rate of approximately 5-10 Hz. Theprocedure has been referred to as “corneal sculpting.”

[0004] Before sculpting of the cornea takes place, the epithelium orouter layer of the cornea is mechanically removed to expose Bowman'smembrane on the anterior surface of the stroma. At this point, laserablation at Bowman's layer can begin. An excimer laser beam is preferredfor this procedure. The beam may be variably masked during the ablationto remove corneal tissue to varying depths as necessary for recontouringthe anterior stroma. Afterward, the epithelium rapidly regrows andresurfaces the contoured area, resulting in an optically correct (ormuch more nearly so) cornea. In some cases, a surface flap of the corneais folded aside and the exposed surface of the cornea's stroma isablated to the desired surface shape with the surface flap then beingreplaced.

[0005] Phototherapeutic keratectomy (PTK) is a procedure involvingequipment functionally identical to the equipment required for PRK. ThePTK procedure differs from PRK in that rather than reshaping the cornea,PTK uses the aforementioned excimer laser to treat pathologicalsuperficial, corneal dystrophies, which might otherwise require cornealtransplants.

[0006] In both of these procedures, surgical errors due to applicationof the treatment laser during unwanted eye movement can degrade therefractive outcome of the surgery. The eye movement or eye positioningis critical since the treatment laser is centered on the patient'stheoretical visual axis which, practically-speaking, is approximatelythe center of the patient's pupil. However, this visual axis isdifficult to determine due in part to residual eye movement andinvoluntary eye movement known as saccadic eye movement. Saccadic eyemovement is high-speed movement (i.e., of very short duration, 10-20milliseconds, and typically up to 1° of eye rotation) inherent in humanvision and is used to provide dynamic scene to the retina. Saccadic eyemovement, while being small in amplitude, varies greatly from patient topatient due to psychological effects, body chemistry, surgical lightingconditions, etc. Thus, even though a surgeon may be able to recognizesome eye movement and can typically inhibit/restart a treatment laser byoperation of a manual switch, the surgeon's reaction time is not fastenough to move the treatment laser in correspondence with eye movement.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to providea laser beam delivery and eye tracking method and system that is used inconjunction with a laser system capable of eroding a surface.

[0008] Another object of the present invention is to provide a systemfor delivering a treatment laser to a surface and for automaticallyredirecting the treatment laser to compensate for movement of thesurface.

[0009] Still another object of the present invention is to provide asystem for delivering a corneal ablating laser beam to the surface of aneye in a specific pattern about the optical center of the eye, and forautomatically redirecting the corneal ablating laser beam to compensatefor eye movement such that the resulting ablating pattern is the sameregardless of eye movement.

[0010] Yet another object of the present invention is to provide a laserbeam delivery and eye tracking system for use with an ophthalmictreatment laser where the tracking operation detects eye movement in anon-intrusive fashion.

[0011] A further object of the present invention is to provide a laserbeam delivery and eye tracking system for automatically delivering andmaintaining a corneal ablating laser beam with respect to the geometriccenter of an eye's pupil or a doctor defined offset from the center ofthe eye's pupil. A special object of this invention is the use of thelaser pulses which are distributed in a pattern of discrete ablations toshape objects other than for corneal ablating.

[0012] Other objects and advantages of the present invention will becomemore obvious hereinafter in the specification and drawings.

[0013] In accordance with the present invention, an eye treatment laserbeam delivery and eye tracking system is provided. A treatment laser andits projection optics generate laser light along an original beam path(i.e., the optical axis of the system) at an energy level suitable fortreating the eye. An optical translator shifts the original beam path inaccordance with a specific scanning pattern so that the original beam isshifted onto a resulting beam path that is parallel to the original beampath. An optical angle adjuster changes the resulting beam path's anglerelative to the original beam path such that the laser light is incidenton the eye.

[0014] An eye movement sensor detects measurable amounts of movement ofthe eye relative to the system's optical axis and then generates errorcontrol signals indicative of the movement. The eye movement sensorincludes 1) a light source for generating light energy that isnon-damaging with respect to the eye, 2) an optical delivery arrangementfor delivering the light energy on a delivery light path to the opticalangle adjuster in a parallel relationship with the resulting beam pathof the treatment laser, and 3) an optical receiving arrangement. Theparallel relationship between the eye movement sensor's delivery lightpath and the treatment laser's resulting beam path is maintained by theoptical angle adjuster. In this way, the treatment laser light and theeye movement sensor's light energy are incident on the eye in theirparallel relationship.

[0015] A portion of the eye movement sensor's light energy is reflectedfrom the eye as reflected energy traveling on a reflected light pathback through the optical angle adjuster. The optical receivingarrangement detects the reflected energy and generates the error controlsignals based on the reflected energy. The optical angle adjuster isresponsive to the error control signals to change the treatment laser'sresulting beam path and the eye movement sensor's delivery light path incorrespondence with one another. In this way, the beam originating fromthe treatment laser and the light energy originating from the eyemovement sensor track along with the eye's movement.

[0016] In carrying out this technique, the pattern constitutesoverlapping but not coaxial locations for ablation to occur with eachpulse removing a microvolume of material by ablation or erosion. Fordifferent depths, a pattern is repeated over those areas where increasedablation is needed. The laser pulses are usually at a certain pulserepetition rate. The subsequent pulses in a sequence are spaced at leastone pulse beam width from the previous pulse and at a distance theablated particles will not substantially interfere with the subsequentpulse. In order to maximize the speed of the ablation, the subsequentpulse is spaced sufficiently close to enable the beam to be moved to thesuccessive location within the time of the pulse repetition. Theablation is carried out on an object until a desired specific shape isachieved.

[0017] This technique is fundamentally new and may be used on objectsother than corneas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram of a laser beam delivery and eyetracking system in accordance with the present invention as it would beused in conjunction with an ophthalmic treatment laser;

[0019]FIG. 2 is a sectional view of the projection optics used with theophthalmic treatment laser embodiment of the laser beam delivery portionof the present invention;

[0020]FIG. 3 illustrates diagrammatically an optical arrangement ofmirrors used to produce translational shifts in a light beam along oneaxis;

[0021]FIG. 4 is a block diagram of the servo controller/motor drivercircuitry used in the ophthalmic treatment laser embodiment of thepresent invention; and

[0022]FIG. 5 is a block diagram of a preferred embodiment eye movementsensor used in the ophthalmic treatment laser embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Referring now to the drawings, and more particularly to FIG. 1, ablock diagram is shown of a laser beam delivery and eye tracking systemreferenced generally by the numeral 5. The laser beam delivery portionof system 5 includes treatment laser source 500, projection optics 510,X-Y translation mirror optics 520, beam translation controller 530,dichroic beamsplitter 200, and beam angle adjustment mirror optics 300.By way of example, it will be assumed that treatment laser 500 is a 193nanometer wavelength excimer laser used in an ophthalmic PRK (or PTK)procedure performed on a movable workpiece, e.g., eye 10. However, it isto be understood that the method and system of the present inventionwill apply equally as well to movable workpieces other than an eye, andfurther to other wavelength surface treatment or surface eroding lasers.The laser pulses are distributed as shots over the area to be ablated oreroded, preferably in a distributed sequence. A single laser pulse ofsufficient power to cause ablation creates a micro cloud of ablatedparticles which interferes with the next laser pulse if located in thesame or immediate point. To avoid this interference, the next laserpulse is spatially distributed to a next point of erosion or ablationthat is located a sufficient distance so as to avoid the cloud ofablated particles. Once the cloud is dissipated, another laser pulse ismade adjacent the area prior eroded so that after the pattern of shotsis completed the cumulative shots fill in and complete said pattern sothat the desired shape of the object or cornea is achieved.

[0024] In operation of the beam delivery portion of system 5, lasersource 500 produces laser beam 502 which is incident upon projectionoptics 510. Projection optics 510 adjusts the diameter and distance tofocus of beam 502 depending on the requirements of the particularprocedure being performed. For the illustrative example of an excimerlaser used in the PRK or PTK procedure, projection optics 510 includesplanar concave lens 512, and fixed focus lenses 514 and 516 as shown inthe sectional view of FIG. 2. Lenses 512 and 514 act together to form anA-focal telescope that expands the diameter of beam 502. Fixed focuslens 516 focuses the expanded beam 502 at the workpiece, i.e., eye 10,and provides sufficient depth, indicated by arrow 518, in the plane offocus of lens 516. This provides flexibility in the placement ofprojection optics 510 relative to the surface of the workpiece. Analternative implementation is to eliminate lens 514 when lessflexibility can be tolerated.

[0025] After exiting projection optics 510, beam 502 impinges on X-Ytranslation mirror optics 520 where beam 502 is translated or shiftedindependently along each of two orthogonal translation axes as governedby beam translation controller 530. Controller 530 is typically aprocessor programmed with a predetermined set of two-dimensionaltranslations or shifts of beam 502 depending on the particularophthalmic procedure being performed. For the illustrative example ofthe excimer laser used in a PRK or PTK procedure, controller 530 may beprogrammed in accordance with the aforementioned copending patentapplication entitled “Laser Sculpting System and Method”. The programmedshifts of beam 502 are implemented by X-Y translation mirror optics 520.

[0026] Each X and Y axis of translation is independently controlled by atranslating mirror. As shown diagrammatically in FIG. 3, theY-translation operation of X-Y translation mirror optics 520 isimplemented using translating mirror 522. Translating mirror 522 ismovable between the position shown and the position indicated by dottedline 526. Movement of translating mirror 522 is such that the angle ofthe output beam with respect to the input beam remains constant. Suchmovement is brought about by translation mirror motor and control 525driven by inputs received from beam translation controller 530. By wayof example, motor and control 525 can be realized with a motor fromTrilogy Systems Corporation (e.g., model T050) and a control board fromDelta Tau Systems (e.g., model 400-602276 PMAC).

[0027] With translating mirror 522 positioned as shown, beam 502 travelsthe path traced by solid line 528 a. With translating mirror 522positioned along dotted line 526, beam 502 travels the path traced bydotted line 528 b. A similar translating mirror (not shown) would beused for the X-translation operation. The X-translation operation isaccomplished in the same fashion but is orthogonal to the Y-translation.The X-translation may be implemented prior or subsequent to theY-translation operation.

[0028] The eye tracking portion of system 5 includes eye movement sensor100, dichroic beamsplitter 200 and beam angle adjustment mirror optics300. Sensor 100 determines the amount of eye movement and uses same toadjust mirrors 310 and 320 to track along with such eye movement. To dothis, sensor 100 first transmits light energy 101-T which has beenselected to transmit through dichroic beamsplitter 200. At the sametime, after undergoing beam translation in accordance with theparticular treatment procedure, beam 502 impinges on dichroicbeamsplitter 200 which has been selected to reflect beam 502 (e.g., 193nanometer wavelength laser beam) to beam angle adjustment mirror optics300.

[0029] Light energy 101-T is aligned such that it is parallel to beam502 as it impinges on beam angle adjustment mirror optics 300. It is tobe understood that the term “parallel” as used herein includes thepossibility that light energy 101-T and beam 502 can be coincident orcollinear. Both light energy 101-T and beam 502 are adjusted incorrespondence with one another by optics 300. Accordingly, light energy101-T and beam 502 retain their parallel relationship when they areincident on eye 10. Since X-Y translation mirror optics 520 shifts theposition of beam 502 in translation independently of optics 300, theparallel relationship between beam 502 and light energy 101-T ismaintained throughout the particular ophthalmic procedure.

[0030] Beam angle adjustment mirror optics consists of independentlyrotating mirrors 310 and 320. Mirror 310 is rotatable about axis 312 asindicated by arrow 314 while mirror 320 is rotatable about axis 322 asindicated by arrow 324. Axes 312 end 322 are orthogonal to one another.In this way, mirror 310 is capable of sweeping light energy 101-T andbeam 502 in a first plane (e.g., elevation) while mirror 320 is-capableof independently sweeping light energy 101-T-and beam 502 in a secondplane (e.g., azimuth) that is perpendicular to the first plane. Uponexiting beam angle adjustment mirror optics 300, light energy 101-T andbeam 502 impinge on eye 10.

[0031] Movement of mirrors 310 and 320 is typically accomplished withservo controller/motor drivers 316 and 326, respectively. FIG. 4 is ablock diagram of a preferred embodiment servo controller/motor driver316 used for the illustrative PRK/PTK treatment example. (The samestructure is used for servo controller/motor driver 326.) In general,drivers 316 and 326 must be able to react quickly when the measurederror from eye movement sensor 100 is large, and further must providevery high gain from low frequencies (DC) to about 100 radians per secondto virtually eliminate both steady state and transient error.

[0032] More specifically, eye movement sensor 100 provides a measure ofthe error between the center of the pupil (or an offset from the centerof the pupil that the doctor selected) and the location where mirror 310is pointed. Position sensor 3166 is provided to directly measure theposition of the drive shaft (not shown) of galvanometer motor 3164. Theoutput of position sensor 3166 is differentiated at differentiator 3168to provide the velocity of the drive shaft of motor 3164.

[0033] This velocity is summed with the error from eye movement sensor100. The sum is integrated at integrator 3160 and input to currentamplifier 3162 to drive galvanometer motor 3164. As the drive shaft ofmotor 3164 rotates mirror 310, the error that eye movement sensor 100measures decreases to a negligible amount. The velocity feedback viaposition sensor 3166 and differentiator 3168 provides servocontroller/motor driver 316 with the ability to react quickly when themeasured sensor error is large.

[0034] Light energy reflected from eye 10, as designated by referencenumeral 101-R, travels back through optics 300 and beamsplitter 200 fordetection at sensor 100. Sensor 100 determines the amount of eyemovement based on the changes in reflection energy 101-R. Error controlsignals indicative of the amount of eye movement are fed back by sensor100 to beam angle adjustment mirror optics 300. The error controlsignals govern the movement or realignment of mirrors 310 and 320 in aneffort to drive the error control signals to zero. In doing this, lightenergy 101 -T and beam 502 are moved in correspondence with eye movementwhile the actual position of beam 502 relative to the center of thepupil is controlled by X-Y translation mirror optics 520.

[0035] In order to take advantage of the properties of beamsplitter 200,light energy 101-T must be of a different wavelength than that oftreatment laser beam 502. The light energy should preferably lie outsidethe visible spectrum so as not to interfere or obstruct a surgeon's viewof eye 10. Further, if the present invention is to be used in ophthalmicsurgical procedures, light energy 101-T must be “eye safe” as defined bythe American National Standards Institute (ANSI). While a variety oflight wavelengths satisfy the above requirements, by way of example,light energy 101-T is infrared light energy in the 900 nanometerwavelength region. Light in this region meets the above noted criteriaand is further produced by readily available, economically affordablelight sources. One such light source is a high pulse repetition rateGaAs 905 nanometer laser operating at 4 kHz which produces an ANSIdefined eye safe pulse of 10 nanojoules in a 50 nanosecond pulse.

[0036] A preferred embodiment method for determining the amount of eyemovement, as well as eye movement sensor 100 for carrying out such amethod, are described in detail in the aforementioned copending patentapplication. However, for purpose of a complete description, sensor 100will be described briefly with the aid of the block diagram shown inFIG. 2. Sensor 100 may be broken down into a delivery portion and areceiving portion. Essentially, the delivery portion projects lightenergy 101-T in the form of light spots 21, 22, 23 and 24 onto aboundary (e.g., iris/pupil boundary 14) on the surface of eye 10. Thereceiving portion monitors light energy 101-R in the form of reflectionscaused by light spots 21, 22, 23 and 24.

[0037] In delivery, spots 21 and 23 are focused and positioned on axis25 while spots 22 and 24 are focused and positioned on axis 26 as shown.Axes 25 and 26 are orthogonal to one another. Spots 21, 22, 23 and 24are focused to be incident on and evenly spaced about iris/pupilboundary 14. The four spots 21, 22, 23 and 24 are of equal energy andare spaced evenly about and on iris/pupil boundary 14. This placementprovides for two-axis motion sensing in the following manner. Each lightspot 21, 22, 23 and 24 causes a certain amount of reflection at itsposition on iris/pupil boundary 14. Since boundary 14 moves incoincidence with eye movement, the amount of reflection from light spots21, 22, 23 and 24 changes in accordance with eye movement. By spacingthe four spots evenly about the circular boundary geometry, horizontalor vertical eye movement is detected by changes in the amount ofreflection from adjacent pairs of spots. For example, horizontal eyemovement is monitored by comparing the combined reflection from lightspots 21 and 24 with the combined reflection from light spots 22 and 23.In a similar fashion, vertical eye movement is monitored by comparingthe combined reflection from light spots 21 and 22 with the combinedreflection from light spots 23 and 24.

[0038] More specifically, the delivery portion includes a 905 nanometerpulsed diode laser 102 transmitting light through optical fiber 104 toan optical fiber assembly 105 that splits and delays each pulse fromlaser 102 into preferably four equal energy pulses. Assembly 105includes one-to-four optical splitter 106 that outputs four pulses ofequal energy into optical fibers 108, 110, 112, 114. In order to use asingle processor to process the reflections caused by each pulsetransmitted by fibers 108, 110, 112 and 114, each pulse is uniquelydelayed by a respective fiber optic delay line 109, 111, 113 and 115.For example, delay line 109 causes a delay of zero, i.e., DELAY=Ox wherex is the delay increment; delay line 111 causes a delay of x, i.e.,DELAY=Ix; etc.

[0039] The pulse repetition frequency and delay increment x are chosenso that the data rate of sensor 100 is greater than the speed of themovement of interest. In terms of saccadic eye movement, the data rateof sensor 100 must be on the order of at least several hundred hertz.For example, a sensor data rate of approximately 4 kHz is achieved by 1)selecting a small but sufficient value for x to allow processor 160 tohandle the data (e.g., 160 nanoseconds), and 2) selecting the timebetween pulses from laser 102 to be 250 microseconds (i.e., laser 102 ispulsed at a 4 kHz rate).

[0040] The four equal energy pulses exit assembly 105 via optical fibers116, 118, 120 and 122 which are configured as a fiber optic bundle 123.Bundle 123 arranges the optical fibers such that the center of eachfiber forms the corner of a square. Light from assembly 105 is passedthrough an optical polarizer 124 that outputs horizontally polarizedlight beams as indicated by arrow 126. Horizontally polarized lightbeams 126 pass to focusing optics 130 where spacing between beams 126 isadjusted based on the boundary of interest. Additionally, a zoomcapability (not shown) can be provided to allow for adjustment of thesize of the pattern formed by spots 21, 22, 23 and 24. This capabilityallows sensor 100 to adapt to different patients, boundaries, etc.

[0041] A polarizing beam splitting cube 140 receives horizontallypolarized light beams 126 from focusing optics 130. Cube 140 isconfigured to transmit horizontal polarization and reflect verticalpolarization. Accordingly, cube 140 transmits only horizontallypolarized light beams 126 as indicated by arrow 142. Thus, it is onlyhorizontally polarized light that is incident on eye 10 as spots 21, 22,23 and 24. Upon reflection from eye 10, the light energy is depolarized(i.e., it has both horizontal and vertical polarization components) asindicated by crossed arrows 150.

[0042] The receiving portion first directs the vertical component of thereflected light as indicated by arrow 152. Thus, cube 140 serves toseparate the transmitted light energy from the reflected light energyfor accurate measurement. The vertically polarized portion of thereflection from spots 21, 22, 23 and 24, is passed through focusing lens154 for imaging onto an infrared detector 156. Detector 156 passes itssignal to a multiplexing peak detecting circuit 158 which is essentiallya plurality of peak sample and hold circuits, a variety of which arewell known in the art. Circuit 158 is configured to sample (and hold thepeak value from) detector 156 in accordance with the pulse repetitionfrequency of laser 102 and the delay x. For example, if the pulserepetition frequency of laser 102 is 4 kHz, circuit 158 gathersreflections from spots 21, 22, 23 and 24 every 250 microseconds.

[0043] The values associated with the reflected energy for each group offour spots (i.e., each pulse of laser 102) are passed to a processor 160where horizontal and vertical components of eye movement are determined.For example let R21, R22, R23 and R24 represent the detected amount ofreflection from one group of spots 21, 22, 23 and 24, respectively. Aquantitative amount of horizontal movement is determined directly fromthe normalized relationship$\frac{\left( {R_{21} + R_{24}} \right) - \left( {R_{22} + R_{23}} \right)}{R_{21} + R_{22} + R_{23} + R_{24}}$

[0044] while a quantitative amount of vertical movement is determineddirectly from the normalized relationship$\frac{\left( {R_{21} + R_{24}} \right) - \left( {R_{23} + R_{24}} \right)}{R_{21} + R_{22} + R_{23} + R_{24}}$

[0045] Note that normalizing (i.e., dividing by R₂₁+R₂₂+R₂₃+R₂₄) reducesthe effects of variations in signal strength. Once determined, themeasured amounts of eye movement are sent to beam angle adjustmentmirror optics 300.

[0046] The advantages of the present invention are numerous. Eyemovement is measured quantitatively and used to automatically redirectboth the laser delivery and eye tracking portions of the systemindependent of the laser positioning mechanism. The system operateswithout interfering with the particular treatment laser or the surgeonperforming the eye treatment procedure.

[0047] Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in the light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

That which is claimed is:
 1. A method of correcting vision by ablatingcorneal tissue comprising the steps of: providing a laser beam forablating corneal tissue; the laser beam providing laser beam shots, eachshot having a size, duration, and center point; selecting an area of thecornea for ablation; determining the size of the laser shots so that theshots are smaller than the area of the cornea selected for ablation;selecting a pattern and sequence for the placement of the laser beamshots on the selected area of the cornea to obtain the removal of adesired amount of corneal tissue; placing the laser beam shots on theselected area of the cornea in the selected pattern and sequence; theplaced laser beam shots ablating corneal tissue; subsequent shots in thepattern having their centers spaced at least one shot size apart; thepattern and sequence selected such that no laser shots are fired atconsecutive locations and such that no consecutive shots overlap; and,the pattern and sequence selected to allow the eye to clear in one placebefore contacting that place again with another laser shot.
 2. A methodof correcting vision by ablating corneal tissue comprising the steps of:providing a laser beam; the laser beam providing a laser beam shot, theshot having a size, duration, and center point; selecting an area of thecornea; the size of the laser shot less than the selected area of thecornea; placing at least three shots on the selected area of the corneain a predetermined pattern and sequence; the laser shots ablatingcorneal tissue; subsequent shots in the pattern having their centersspaced apart; the pattern and sequence selected such that no laser shotsare fired at consecutive locations and such that no consecutive shotsoverlap; the distance and time between the center point and placement ofthe first laser shot and the center point and placement of the secondlaser shot being sufficient so that any plume of ablated material fromthe first laser shot will not substantially interfere with the secondlaser shot; the distance and time between the center point and placementof the second laser shot and the center and placement of the third lasershot being sufficient so that any plume of ablated material from thesecond laser shot will not substantially interfere with the third lasershot; and, whereby the predetermined pattern and sequence of shotplacement allows the eye to clear in one place before contacting thatplace again with another laser shot.
 3. A method of correcting vision byablating corneal tissue comprising the steps of: providing a laser beam;the laser beam providing laser beam shots, each shot having a size,duration, and center point; selecting an area of the cornea forablation; selecting a pattern and sequence for the placement of thelaser beam shots on the selected area of the cornea to obtain theremoval of a desired amount of corneal tissue; placing the laser beamshots on the selected area of the cornea in the selected pattern andsequence; the laser shots ablating corneal tissue; subsequent shots inthe pattern having their centers spaced apart; the pattern and sequenceselected such that no laser shots are fired at consecutive locations andsuch that no consecutive shots overlap; the pattern and sequenceselected to allow the eye to clear in one place before contacting thatplace again with another laser shot; tracking movements of the selectedarea of the cornea; and, adjusting the placement of the shots on theselected area of the cornea to compensate for the tracked movements ofthe selected area.
 4. A method of correcting vision by ablating cornealtissue comprising the steps of: providing a laser beam; the laser beamproviding laser beam shot, the shot having a size, duration, and centerpoint; selecting an area of the cornea of an eye; the size of the lasershot less than the area of the cornea selected for ablation; placing atleast three shots on the selected area of the cornea in a predeterminedpattern and sequence; the laser shots ablating corneal tissue;subsequent shots in the pattern having their centers spaced apart; thepattern and sequence selected such that no laser shots are fired atconsecutive locations and such that no consecutive shots overlap; thedistance and time between the center point and placement of the firstlaser shot and the center point and placement of the second laser shotbeing sufficient so that any plume of ablated material from the firstlaser shot will not substantially interfere with the second laser shot;the distance and time between the center point and placement of thesecond laser shot and the center and placement of the third laser shotbeing sufficient so that any plume of ablated material from the secondlaser shot will not substantially interfere with the third laser shot;the pattern and sequence of shot placement selected so as to allow theeye to clear in one place before contacting that place again withanother laser shot; tracking movements of the eye; and adjusting theplacement of the shots on the selected area of the cornea to compensatefor the tracked movements of the eye.